A TYPO3 instance is affected if an extension loads single CSS files from EXT:t3skin. Backend modules of
extensions usually get CSS core stuff loaded by default, which will not be a problem. An extension is
only affected if single CSS files are explicitly referenced. This should be a rare case.

Most simple solution is to copy over the "old" CSS file from an older instance. Directly including those
files from t3skin is discouraged. A better solution is to refactor the extension to use the full t3skin
CSS file and to overlay it with required changes in an own file.